Technical Field
The present invention relates to measuring device degradation and, more particularly, to measuring degradation characteristics of individual devices when a circuit degrades from normal or stressed operation.
Description of the Related Art
Degradation of the electrical properties of field complementary metal-oxide semiconductor (CMOS) field effect transistors (FETs) is a consequence of their operation. While efforts have been made to minimize the degradation, circuits are still degraded by a significant and measurable amount in operation, and therefore prediction of the amount of degradation which will occur during the expected lifetime of a device is still needed.
Bias-temperature instability (BTI) produces increases in the threshold voltage of a semiconductor device and therefore reduces the drain current of the device. The progression of BTI degradation is determined by total power-on time of the device and is a result of charge carriers tunneling through barriers and damaging the material along interfaces. For example, in some devices, such as p-type FETs, BTI degrades the conducting channel-to-dielectric interface, creating interface traps that reduce transistor performance. A second degradation mechanism is hot-carrier injection (HCI). In contrast to BTI, HCI degradation is controlled by switching the device's state. HCI is caused by “hot” electrons that have enough energy to interact with the gate dielectric. In both cases, BTI and HCI cause physical damage to the device and will eventually change the properties of the device. Both types of degradation occur naturally during operation of the devices at normal conditions, and can be made to occur more rapidly by raising the temperature and voltages applied to the devices. Application of voltage which results in device degradation is commonly referred to as “stress.”
The degradation of individual FETs results in reduced propagation speed in logic circuits. Hence, ring oscillators (ROs) are often used to measure and predict the degradation, as such degradation results in a reduction of the RO frequency. However, the net frequency reduction does not reveal which particular transistors (e.g., n-type or p-type FETs) in a circuit have aged the most, nor does it reveal what the changes in their characteristics are, nor whether the mechanism is predominately reverse bias-temperature instability (BTI) or hot carrier injection (HCI) damage.